Bit Budget Allocation

How Much Video Will Fit on My DVD?

Many producers ask the question “How much video can we put on this project?” Why do they need this information early on in the production process? Quite simply, they want the best quality the can possibly get out of their DVD without costs getting out of hand. There is a balance between capacity, quality and cost that must be addressed for each DVD.

DVD Capacity

DVDs come in a number of physical configurations, and each is dependent on the amount of storage space available on the disc. A DVD can have a single layer of data on one side. This is known as a DVD-5, because each DVD layer can hold approximately 5GB of data. Technically, this number is 4.7GB for a single-layer DVD. DVDs can also have two layers. Your DVD player can adjust the focus of its laser so that it reads either the first layer (Layer 0) or the second (Layer 1).

Dual-layer DVDs have a slightly smaller capacity for storage per layer than a single-layer DVD, equaling about 4.3GB per layer. For this reason, a dual-layer DVD is called a DVD-9 (4.3 GB x 2 = approx. 9GB). A complete listing of DVDs and their capacities can be found below.

Designation

Layout

Capacity

DVD-5

Single Side, Single Layer

4.70GB

DVD-9

Single Side, Dual Layer

8.54GB

DVD-10

Double Side, Single Layer

9.40GB

DVD-14

Double Side, Single Layer(Dual Layer on One Side)

13.24GB

DVD-18

Double Side, Dual Layer

17.08GB

Note: these capacities are for 12cm DVDs. The DVD Specification also supports 8cm DVDs, with capacities much smaller than those listed here.

From the manufacturing standpoint, the greater capacity the disc, the greater the cost to replicate. This is because each layer must have a physical metal master stamper created for manufacturing. The more masters, the higher the cost.

So manufacturing costs are critical to projecting a budget for a DVD release. So is quality. DVD is a high-quality release format, but it’s possible to lower the quality of a DVD in order to fit within the disc’s “bit budget”.

Bit budget refers to the balance between the amount of video content and the disc’s capacity. Because we can vary the data rate of the video on disc, we can increase the amount of video on a DVD if we sacrifice a little quality.

Video Quality

DVDs have a convenient feature built into them in that they can support a variety of video bitrates. The bitrate is the amount of data per second that is used to play back audio and/or video. We understand there is a limitation to the amount of data that can fit on a disc, but there is also a limit to the number of data bits that can be pulled off of disc per second.

To better appreciate this concept, consider the DVD a bucket full of water, where the water is the visual content. We need a way to pull the data off the disc (the laser), and we’ll consider this as akin to a garden hose attached to the bottom of the bucket. The hose (laser) has a finite diameter. There is a limit to the amount of water that can drain through the hose at any given length of time. Similarly, there is a limit to the amount of data that can fit through the laser at any given length of time.

The maximum sustainable amount of data that can pass through a DVD player’s laser is 9.2Mbit/sec, though in some cases, the data rate can momentarily exceed that rate. As one would expect, the higher the data rate of the video on DVD, the better its quality. There is no theoretical minimum bitrate for DVD Video, but a substantially low bitrate video will result in horribly unwatchable content.

Video quality is not only related to its data rate, but also to its content. Uncompressed video takes a full-resolution picture roughly thirty times per second. At standard definition resolution, this is roughly 288Mbit/s (assuming we’re using 8-bit RGB video). That’s over 30 times the peak bitrate for DVD. It’s no wonder we need to compress video to get DVDs to play. MPEG-2 video, when compressed for DVD, is much smaller than that. Unfortunately when you compare 288Mbits to 9.2Mbits, it quickly becomes clear that something in the compression stage is thrown away.

MPEG video, though a very good codec, displays much less data than full resolution video. Its implementation is pretty nifty , and its success is program-dependent. That is to say, the more motion that takes place on video, the more it requires a high bitrate to properly display all the motion. Compressing video with less motion requires a much lower bitrate to faithfully mimic its uncompressed source than compressing video with more motion. Video of a canoer slowly paddling on a lake is less likely to “break up” onscreen than is video of an explosion or a live concert.

So, the amount of high-quality video that can fit on a DVD varies quite a lot. In general, we try to keep the following guidelines in mind:

Concert footage and high-action sequences require high bitrates. Music Videos and studio performances can survive compression with moderate bitrates. Still images and video sequences with little motion can usually survive low bitrate compression.

Audio Considerations

When planning for a DVD, don’t forget audio. Like video, audio quality can vary on a DVD, though there are fewer complexities, and its data rate is much lower than video. There are 3 types of audio that are compatible with DVD: Dolby Digital, DTS and uncompressed PCM audio. Their bitrates range from 192kbit/s to 4.6Mbit/s. Generally, we use Dolby Digital at 448kbit/s or DTS Surround at 1.5Mbit/s. The total allowable bitrate for a DVD includes both the video bitrate and the audio bitrate combined, so this must be factored in when planning a DVD’s bit budget.

Other Factors

Not only do audio and video play important roles in determining the amount of storage space available on your DVD, but you will also need to consider menus, any files you may want to store in the ROM zone (a computer-compatible portion of the disc that can store nearly any type of computer-useable files) and the layer break. When you include still menus (photo galleries, credits or any other extra menus), you use more disc space. Any computer files will use disc space, and some extra space is allocated when we choose the proper point at which to assign the layer break. That’s the point at which a dual-layer DVD moves from one layer to the next (it has to be programmed in authoring).

General Scenarios

The table below outlines some general scenarios for a DVD and breaks down the possible combination of options for single-layer and dual-layer discs, optimizing for video and audio quality, using two audio streams and leaving some space for still menus (but no ROM data). This is by no means a complete or absolute list of possibilities.

Type of Video

Audio Quality

DVD-5

DVD-9

Live Concert/Action Film

Dolby Digital

60 min

110 min

Live Concert/Action Film

DTS Audio*

60 min

105 min

Live Concert/Action Film

Dolby Digital/PCM*

55 min

110 min

Studio Performance/Theater

Dolby Digital

80 min

145 min

Studio Performance/Theater

DTS Audio

70 min

125 min

Studio Performance/Theater

Dolby Digital/PCM

65 min

110 min

Low Motion / Slide Shows

Dolby Digital

105 min

180 min

Low Motion / Slide Shows

DTS Audio

85 min

150 min

Low Motion / Slide Shows

Dolby Digital/PCM

75 min

135 min

* Indicates that video bitrate must be reduced in order to accommodate high audio bitrates.